The orientation of the image in MRI is controlled by varying the main magnetic field using gradient coils. In conducting MRI experiments, the coil must be tuned to the resonant frequency of the nuclei to be observed. Additionally, the impedance of the coil should be electrically matched to the impedance of the transmission line which is optimally coupled through the multiplexer to the receiver to obtain the maximum transfer of energy and to obtain the best signal to noise ratio (SNR). To tune and match the coil, conventional NMR coils have variable capacitors. Typically, at least one variable capacitor is adjusted to tune the coil to the desired resonant frequency and at least another variable capacitor is adjusted to match the impedance of the coil. To adjust the capacitance of the variable capacitors, mechanical linkages are coupled to variable capacitors in the coil known as the tuning rods.
The tuning rods extend from the proximal end of the transmit coil so as to allow a user to adjust the variable capacitors when the transmit coil is installed in the gradient coil. Mechanical clutches mounted on the tuning rods allow the tuning rods to slip when the variable capacitors reach the end of their adjustment range, thus reducing the likelihood of damage to the capacitors should the user attempt to turn a tuning rod past a capacitor's adjustment range.
A clutch is defined as a mechanical device that provides for the transmission of power (and therefore usually motion) from one component (the driving member) to another (the driven member) when engaged, but can be disengaged. A slip clutch is allows a rotating shaft to slip when higher than normal resistance is encountered on a machine.
Several MRI devices recite a clutch. For example, U.S. Pat. No. 6,323,647 recites motor driven tuning and matching of RF coils in an NMR probe comprising a switching clutch that includes a wave washer. Patent application US2008/0204028 also recites a mechanical clutch to allow the tuning rods to slip when the variable capacitors reach the end of their adjustment range, thus reducing the likelihood of damage to the capacitors.
However, no clutch was designed specifically for connecting the tuning rods to the capacitor whereas taking into consideration the special conditions of MRI safety.
There is thus a long unfelt need for a clutch that is MRI safe specifically designed to connected the tuning rods of the MRI coil to the capacitor.